1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet, and to image reading apparatus and image forming apparatus provided therewith.
2. Related Background Art
As automatic original feeding apparatus there are apparatus of an original circulating method (RDF) and a non-circulating method (ADF). In the former RDF case, there are apparatus for completing exposure while an original passes an exposure section, and discharging the original onto a region above (or below) an original mounting section; further, in the case of setting of plural sets of copies of originals, there are apparatus for obtaining plural sets of copies by successively recirculating the originals.
The combination of RDF with the flow reading mode for finishing exposure during a moving state of an original as described is free of the loss time corresponding to the moving time of exposure apparatus as against a fixed reading mode for first stopping the original and then moving the exposure apparatus along an original region. Therefore, the combination has advantages that it contributes to shortening of original changing time and, in turn, to speed increasing technology and productivity increasing technology of the copying system and that, when a comparison is made in a predetermined original changing time, it enables the original to be circulated at lower moving speed than in the fixed reading mode, thereby also contributing to quieting (noise reducing) technology.
First describing examples of the combination with RDF, there also exist systems able to achieve the speed increase and productivity increase of the copying system by combination with flash exposure technology. The detailed description of the flash exposure technology is omitted herein, but, when compared with the aforementioned exposure apparatus, it employs structure for completing exposure of all surfaces of an original at one time and needs to use a high-power light source and a belt type photosensitive member, thus increasing the size of apparatus, the cost, and electric power consumption. It is, therefore, the present status that applications of this technology are limited to some large-scale high-speed machines.
The latter ADF apparatus has such structure that an original is fixed at a predetermined position on a platen, exposure of copying section is repeated a necessary copy set number of times by translationally moving the exposure apparatus, the original is discharged onto a predetermined discharge tray after completion of exposure, and this operation is successively repeated to obtain plural sets of copies from a series of originals. The copying system is combined with a sorter provided on the output side thereof, which eliminates the need for repetitively circulating the originals many times. Therefore, this apparatus has an advantage of relatively small damage of original.
Under the above circumstances of existence of the conventional technology, a variety of proposals have been made in order to achieve the speed increase and quieting in recent years.
A first example of the apparatus devised is a sheet medium conveying apparatus wherein the originals on a sheet mounting table are fed by automatically selecting one of two feeding means provided at both ends of the sheet mounting table, depending upon the conditions including the size of original and a set mode, and an original is conveyed up to an image reading section of the copier body to be read there.
For example, an original-recirculatable system is arranged, as shown in FIG. 8, so that in the original fixed reading mode (for feeding the original through a first feeding means) a plurality of sheet originals P loaded on a sheet original mounting table 304 are separated one by one by first separating means 306, they are conveyed by feeding means 338 to be successively loaded at an arbitrary position on platen 303, an image of each original is read while moving an image reading section (optical system) 380 in the direction a in the copier body 301, and the original is reloaded on the sheet mounting table 304 by discharge means 311. In the original flow reading mode (for feeding the original through a second feeding means) the plurality of sheet originals P loaded on the original tray 304 are separated one by one by second separating means 314 to be conveyed by feeding means 315, the optical system 380 is fixed at a position the distance L apart from the home position (illustrated by a dashed line) of optical system 380, an image is read while the sheet original P is conveyed at constant speed on the platen 303 by a wide belt 307, and the original is loaded onto the sheet original mounting table 304 by the discharge means 311.
When the flow reading mode is selected, for example, under such conditions that a user puts originals of a small size (A4, B5, LTR, or the like) on the original tray 304, a start key (not illustrated) of the copier body is on, a first entrance sensor 322 is on, and a sheet medium length detecting sensor 368 is off, the operation advances in the flow reading mode. First, the original tray 304 falls down to a predetermined position about the fulcrum of 340, a stopper (sheet stack conveying means) 321 conveys the stack of sheet originals P toward the second separating means 314 to move the stack up to the position of stack conveying position detecting sensor 328 (before the stack conveying position detecting sensor 328 becomes on), feeding from the second feeding means 315 is started to feed the original to the platen 303, the image of the original is read in the aforementioned original flow reading mode, and then the sheet original is discharged from the discharge roller 311 onto the original tray 304. The stopper 321 pushes the rear end of sheet original toward the second separating section every discharge of original so as to enhance alignment. For one more circulation, after the all sheet originals are discharged, the stack is conveyed together to be again fed and to be again copied.
The above devised apparatus was suitable for the productivity increase, because it is the original recirculating method processing apparatus to achieve the high original changing speed in the flow reading mode, and also suitable for the quieting, as described above; however, it required circulation of originals of a setting number times for obtaining plural copies thereof. The apparatus thus had the structure requiring the advanced technologies for realizing the system that gave less damage on the originals, for example, by decreasing the stress on the originals upon passage through the separating means of the above apparatus, curling of original upon passage through the bent path, and so on, and that had high reliability by decreasing the probability of conveyance failure upon handling of original.
Particularly, in the above apparatus, the separating and feeding means of original P employs the bottom separating and feeding method of a conventional belt retard system comprised of a retard belt and a feeding roller, as shown in FIG. 8. This method is a method for aligning the originals at their leading edges and feeding them, and it thus has an advantage that even a stack of originals with different sizes mixed can be separated and fed. However, it has a problem of soiling of original occurring when a printed surface of the first original P rubs the back surface of the second original P upon feeding from the lowermost of the originals P mounted in the face-up state. Various means have been considered to overcome it, and considerable improvements have been achieved in the soiling.
When the originals P had poorly fixed images and the number of originals P was large, the above soiling was conspicuous when seen in a stack of many originals P after copy. A further improvement was thus demanded. A separating and feeding method using air is known as feeding means for solving the problem of soiling of original. The structure of this air feeding is free of not only the problem of soiling of original but also the problem of abrasion of the belt and rubber in the retard system, and it thus has advantages of excellent durability and reliability.
In the above structure of air feeding it is, however, necessary to prevent accompanied conveyance by making use of the weight of originals P by mounting the originals on the original mounting tray 2 inclined at an angle of rise in the sheet feeding direction toward the air separating means, and to align the originals P at the trailing edges thereof.
Therefore, the air feeding method has a problem that it is not ready for handling a stack of originals P of different sizes mixed, different from the aforementioned retard system.
Further, in the air feeding method, when the originals have holes at the leading edges thereof in the original feeding direction, the separation air directed toward the leading edges of originals leaks through the hole portions, so that the second and after originals become reluctant to float. The air feeding method thus has another problem that it is not suitable for the originals with holes at the leading edges in the original feeding direction.